Dielectric properties of rubber ferrite composites containing mixed ferrites

Rubber ferrite composites (RFC) are important since they have useful applications as microwave absorbers and flexible magnets. The mouldability of these composites into complex shapes is another advantage. The evaluation of their dielectric and magnetic properties is important in understanding the physical properties of these composites. Pre-characterized nickel zinc ferrites (Ni_1-xZn_xFe₂O₄ where 0 x1 in steps of 0.2) prepared by ceramic techniques were incorporated in to a butyl rubber matrix according to a specific recipe to yield RFCs. The dielectric constant of ceramic Ni_1-xZn_xFe₂O₄ and the butyl rubber composites incorporated with Ni_1-xZn_xFe₂O₄ are studied as a function of frequency, composition, loading and temperature. The observed data indicates that the dependence of the dielectric constant on frequency follows Maxwell-Wagner interfacial polarization. The compositional (zinc content, i.e. x value) dependence shows that the dielectric constant increases initially and reaches a maximum value for the composition corresponding to x = 0.6 and thereafter it decreases. This can be explained on the basis of porosity and alternating current (AC) conductivity. It was also observed that the dielectric constant of the composite material increases with an increase of the volume fraction of the magnetic filler. These observations satisfy some mixture equations, which correlate the dielectric constant of the matrix, filler and the composites. The temperature dependence of the dielectric constant of the ceramic samples as well as the RFCs shows an increase with an increase of temperature at low frequencies. The dielectric constant of the blank butyl rubber was also determined. It was observed that for a blank sample (without filler) the dielectric constant decreases with an increase of temperature. This is due to the decrease in polymer density with increase in temperature. These results suggest that the magnetic and dielectric properties of RFCs can be manipulated by appropriate loading and a judicious choice of the magnetic filler. The modification of these properties will aid in the design of composite materials for microwave absorbers.